The use of conjugate polarizations (also known as cross-polarizations) for the purpose of doubling the information-carrying capacity of a given band of microwave frequencies is well known, e.g. from U.S. Pat. No. 3,735,266. A linearly polarized wave may be considered as consisting of two orthogonal vectors in phase with each other whereas with circular polarization the two vectors are in quadrature. It is thus possible, by advancing or retarding the relative phase of these vectors, to convert one type of polarization into the other; the conjugacy of two linearly polarized waves, whose planes of polarization stand at right angles to each other, can therefore readily be preserved with circular polarization by using differential phase shifts of +90.degree. and -90.degree. to establish opposite directions of rotation. The term "conjugate", as herein used, applies accordingly both to oppositely rotating circular polarizations and to mutually perpendicular linear polarizations.
When microwave energy with two conjugate polarizations is transmitted in free space, e.g. between ground stations and a satellite, atmospheric disturbances and the nonhomogeneity of the transmission medium tend to create polarization distortions in the form of relative amplitude changes and phase shifts between the associated vectors which result in undesirable cross-talk between the two half-channels. It has already been proposed to utilize a pair of pilot frequencies or beacon signals in these half-channels for deriving therefrom, at the receiving end, information on the existing polarization distortion for the purpose of compensating same. Reference in this connection may be made to an article by T. S. Chu entitled "Restoring the Orthogonality of Two Polarizations in Radio Communication Systems", published in two installments in the Bell System Technical Journal, Vol. 50, No. 9 (pages 3063- 3069) and Vol. 52, No. 3 (pages 319-327). That article also teaches the joint use of a differential attenuator or amplifier and a differential phase shifter as corrective devices.
In copending application Ser. No. 603,232 filed Aug. 8, 1975 by one of us, Enzo Cavalieri D'Oro, there has been disclosed a purely electronic system for correcting polarization distortion in a receiver of dual-polarized microwaves as discussed above. That system comprises two differential amplitude changers (attenuators or amplifiers) and two differential phase shifters, one for each polarization, in a waveguide section conducting incoming microwave energy of the dual-polarized type, the two differential amplitude changers being effective in planes including with each other a first acute angle (preferably of 45.degree.) while the two differential phase shifters are effective in planes including with each other a second acute angle (perferably also of 45.degree.). Channel energy with a first and a second polarization, mutually conjugate as hereinabove defined, is extracted from the waveguide section downstream of the differential amplitude changers and phase shifters by a pair of directive couplers, preceded if necessary by a polarization converter which linearizes the incoming microwaves if the same are circularly polarized. One of the directive couplers, assumed by way of example to be vertically oriented, works into a first discriminator which separates a principal component V of a first pilot frequency and a distortion-induced supplemental component v of a second pilot frequency from accompanying message signals; the other coupler, assumed by way of example to be horizontally oriented, coacts with a second discriminator similarly separating a principal component H of the second pilot frequency and a distortion-induced supplemental component h of the first pilot frequency from message signals accompanying same. A processor with input connections from the first and second discriminators and with output connections to the differential amplitude changers and phase shifters obtains from the components V, v, H and h four control signals for respectively adjusting the differential amplitude changers and phase shifters to modify the corrective amplitude and phase distortions introduced thereby into the two half-channels, with the effect of minimizing the supplemental components h and v.
As further described in the copending application, the same control signals may be fed to similar amplitude changers and phase shifters in a waveguide section for introducing compensatory amplitude and phase distortions in two outgoing half-channels.
The above-described distortion compensator may be characterized as of the regenerative type, in contradistinction to the suppressive type of distortion corrector which operates by canceling out the undesired signal components. Regenerative compensation, carried out at microwave frequency within a receiving waveguide, is effective mainly against distortion which more or less uniformly affects the entire transmission band. This kind of distortion results mainly from the anisotropy of the transmitting medium due to atmospherical and ionospherical phenomena such as rain, air currents and Faraday rotation. There are, however, other factors which exert a nonuniform influence upon different sectors of the frequency band and therefore upon the several message channels into which such a band may be divided. While the technique of regenerative compensation could be applied separately to the individual message channels, such a system would be rather costly and somewhat uneconomical in view of the complexity of the circuits requiring for each channel a pair of differential amplitude changers and a pair of differential phase shifters as described above. On the other hand, the use of suppressive distortion correction in the high-frequency section of a receiver tends to reduce the signal-to-noise ratio to a significant extend and is also rather ineffectual against wide-band distortions due to ambient conditions.